An accurate knowledge of spectral properties (for example, a bandwidth) of an optical source such as a laser is important in many scientific and industrial applications. For example, accurate knowledge of the optical source bandwidth is needed to enable control of a minimum feature size or critical dimension (CD) in deep ultraviolet (DUV) optical lithography. The critical dimension is the feature size that needs to be printed on a semiconductor substrate (also referred to as a wafer) and therefore the CD can require tight size control. In optical lithography, the substrate is irradiated by a light beam produced by an optical source. Often, the optical source is a laser source and the light beam is a laser beam. To enhance resolution of the process and to therefore reduce the minimum feature size, a fluid medium having a refractive index greater than one can fill a gap between a final lens of an illuminator of the apparatus and the substrate.
The bandwidth of a light beam is the width of the intensity spectrum of the light beam output from the optical source, and this width can be given in terms of wavelength or frequency of the laser light. Any suitable mathematical construction (that is, metric) related to the details of the optical source spectrum can be used to estimate the bandwidth of the light beam. For example, the full width of the spectrum at a fraction (X) of the maximum peak intensity (referred to as FWXM) can be used to estimate the light beam bandwidth. As another example, a width of the spectrum that contains a fraction (Y) of the integrated spectral intensity (referred to as EY) can be used to estimate the light beam bandwidth.